Lye

From Dwarf Fortress Wiki

Lye is made with a Lye maker dwarf at an ashery and requires 1 block of ash and 1 bucket. It is used to make soap and potash, though potash can also be made directly from ash. 3 lye must be moved from the buckets into a barrel to make the soap.

Why make potash out of lye instead of ash?

• To train your dwarfs by making them do extra tasks.
• If you have left over lye from making soap.
• Lye can be bought at the "embark" screen but ashes cannot.